composite: Make compIsAlternateVisual safe even if Composite is off

As of ea483af9 we're calling this unconditionally from the GLX code so
the synthetic visual is in a lower select group. If Composite has been
disabled then GetCompScreen() will return NULL, and this would crash.

Rather than force the caller to check first, just always return FALSE if
Composite is disabled (which is correct, since none of the visuals will
be synthetic in that case).

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
(cherry picked from commit f8011912)